50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

The North Island Fault System (NIFS) is the
principal strike-slip fault system of the Hikurangi
margin, along which the Pacific Plate is being
subducted beneath the overriding Australian Plate.
The NIFS accommodates up to 50% of the marginparallel
relative plate motion and extends for
approximately 450 km, from the Wellington region
in the south, to the Bay of Plenty coastline in the
north. For its northern c.120 km, the NIFS
comprises three main sub-parallel strands
(Waiohau, Whakatane & Waimana faults).
Northwards along these faults, late Quaternary rates
of strike-slip decrease as rates of normal-slip
increase, a transition across which horizontal to
vertical slip-ratios decrease from 5-10:1 to 0.6:1
(Mouslopoulou et al., 2004). In this talk we will
identify the position of this kinematic transition on
each of the main faults, and assess whether it may
have functioned as a mechanical barrier to rupture
propagation during the history of paleoearthquakes
known for this region. We use fault-trench log data
from 15 trenches and >50 measurements of offset
landforms. The ages of key stratigraphic markers
are constrained by tephra glass chemistry and 14 C
dating. Preliminary results suggest that the
Whakatane Fault last ruptured since 0.8 kyr BP,
while the Waimana and Waiohau faults last
ruptured 1.8-2.8 and 1.8-5.6 kyr ago, respectively.
Within c. 40 km of the Bay of Plenty coastline,
earthquake recurrence intervals average about 2-
2.5, 3 and 4.5 kyr on the Whakatane, Waimana, and
Waiohau faults, respectively. On individual faults,
the timing and number of earthquakes during the
Holocene appears to have varied along strike across
the kinematic transition zone. On the
Ruahine/Waiohau Fault, for example, the strike-slip
Ruahine Fault generated 6 Holocene earthquakes,
while to the north in the Galatea Basin, where the
fault is predominantly normal-slip, only 1-2
Holocene earthquakes have been recorded
(Beanland, 1995; Hansen, 1997). A decrease in the
number of Holocene earthquakes also appears to
occur northwards along the Whakatane Fault,
across the kinematic transition zone (between
Ruatahuna to Ruatoki North). Although not
conclusive, the data suggest that some
paleoearthquakes on the Whakatane and Waiohau
faults terminated within the kinematic transition
zone, which functioned as a mechanical barrier to
propagation of large ruptures. The along-strike
change in fault kinematics, defined by a 60-70°
northward steepening in the pitch of the slip vector
on the faults, may therefore separate fault segments
which ruptured during strike slip and normal slip
dominated earthquakes.
Beanland, S. 1995. PhD Thesis, Victoria University of
Wellington, New Zealand.
Hansen J., 1998. PhD Thesis. Massey University, New
Zealand.
Mouslopoulou et al., 2004. Geol. Soc. NZ Miscell. Pub.
117A
GSNZ VOICES FROM THE PAST
ORAL
Simon Nathan
Te Ara: Encyclopedia of New Zealand
Ministry for Culture & Heritage, P.O. Box 5364,
Wellington
(simon.nathan*mch.govt.nz)
In 1955 the geological community in New Zealand
was rather different from today. The four university
departments were small, and the majority of
geologists (almost entirely male) worked for the
Geological Survey. The lack of a mining industry
meant that there were few job openings for
graduates. Geology was almost entirely based on
surface outcrops, and there was little knowledge of
offshore geology or the extent of major
hydrocarbon-bearing sedimentary basins.
This talk focuses on some of the individuals who
founded the Geological Society of New Zealand in
1955, and will be illustrated by a selection of voices
from the past.
ORAL
TWIN PEAKS: THE PRE-MANGERE
FORMATION SEQUENCE ON MANGERE
ISLAND, CHATHAM ISLANDS
V.E. Neall, G.Davies&R.C.Wallace
Institute of Natural Resources, Massey University,
Palmerston North
(V.E.Neall*massey.ac.nz)
During early 2005, whilst studies of the Mangere
Formation on Mangere Island were in progress as
part of the CHEARS Marsden project, an
opportunity arose to investigate the pre-Mangere
Formation sequence. This paper re-examines the
pre-Mangere Formation sequence on Mangere
Island and proposes a new Twin Peaks model.
Rangiauria Breccia is mapped as the major volcanic
unit forming the edifice of Mangere and Little
Mangere Islands. Two distinct lithofacies were
originally recognised by Hay et al. (1970), but
subsequently amalgamated by Campbell et al.
(1988). The highly lithified limburgitic breccia
lithofacies forms the high bluffs of northern
Mangere Island and Little Mangere Island. In
contrast, a weakly consolidated limburgitic breccia
lithofacies forms the linear southern flank of
Mangere Island. Orientation of bedding within
these lithofacies points to more than 6 centres of
50 th Kaikoura05 -56- Kaikoura 2005